Capacitors are used extensively in electronic devices for storing an electric charge. A capacitor includes two conductive plates or electrodes separated by an insulator. The capacitance, or amount of charge held by the capacitor per applied voltage, depends upon the area of the plates, the distance between them, and the dielectric value of the insulator. Capacitors may be formed within a semiconductor device, such as, for example, a dynamic random access memory (DRAM) or an embedded DRAM.
As semiconductor memory devices become more highly integrated, the area occupied by the capacitor of a DRAM storage cell is reduced, thus decreasing the capacitance of the capacitor due to a smaller electrode surface area. However, a relatively large capacitance is desired to prevent loss of stored information. Therefore, it is desirable to reduce the cell dimensions and yet obtain a high capacitance, which achieves both high cell integration and reliable operation.
Instead of forming the capacitor on the substrate surface, capacitors are also formed above the substrate, i.e., they are stacked above the substrate. The surface area of the substrate can then be used for forming transistors. For example, U.S. Pat. No. 5,903,493 to Lee discloses a capacitor formed above a tungsten plug. The tungsten plug interfaces with an interconnection line, thus allowing different layers formed above the substrate to be connected. Such plugs may be anchored or tapered to secure the plug in the dielectric layer.
Current 0.25 and 0.2 micron semiconductor technology uses metal-oxide-metal (MOM) capacitors that are formed above tungsten plugs. However, these plugs can have surface defects such as seams, recesses, bulges or other topographical features which may cause MOM capacitor reliability and yield problems. For example, when the dielectric adjacent the tungsten plug is polished during a chemical mechanical polishing (CMP) step, the resulting tungsten plug may protrude or bulge upwardly above the dielectric layer.